**1. Introduction**

Irritable bowel syndrome (IBS) is a common functional gastrointestinal disorder characterized by chronic recurrent abdominal pain, changes in bowel habits, and other symptoms such as bloating and flatulence. Based on the Rome IV criteria, four subtypes of IBS exist depending on the predominant stool pattern, including IBS with constipation (IBS-C), IBS with diarrhea (IBS-D), IBS with mixed bowel habits (IBS-M), and unclassified IBS [1,2]. IBS has a global prevalence of approximately 11% and is associated with several comorbidities, such as anxiety, depression, fibromyalgia, migraines, chronic pelvic pain, and others [3,4]. IBS is a major socioeconomic burden because affected patients utilize more healthcare resources with reduced work productivity when compared to the healthy population [5]. IBS is a complex heterogeneous condition with a multifactorial pathogenesis. Proposed mechanisms involved in the pathogenesis of IBS include visceral hypersensitivity, gut–brain axis alterations, disorders in the epithelial barrier integrity leading to abnormal mucosal intestinal permeability, changed intestinal motility, immune system activation, food intolerance, low-grade inflammation, altered enteroendocrine pathways

signaling, genetic basis (e.g., mutation in the SC5NA gene encoding a sodium channel ion; a number of single-nucleotide polymorphism studies have also identified polymorphisms in genes associated with IBS pathogenesis including genes coding for serotonin signaling, immune regulation, and epithelial barrier function), and the evolving concept of dysbiosis in the gu<sup>t</sup> microbiota (Figure 1) [6–15].

**Figure 1.** Role of the microbiome in irritable bowel syndrome (IBS).

Gastrointestinal (GI) microbiota are the most relevant microbial community in the body and belong to the so-called microbiota which includes all microorganisms living in the human body. The intestinal microbiota includes bacteria, archaea, fungi, eukaryotes, and viruses. Most of the bacteria in the GI tract are represented in four main bacterial phyla: Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria [16,17]. Trillions of microorganisms reside in the GI tract, with the highest density in the colon. However, most of them remain uncharacterized. Since this number is approximately equal to 10 times the total number of body cells, the interest in microbiota study for intestinal and extra-intestinal diseases is not surprising [18–22]. Advances in molecular biology techniques have significantly contributed to microbiome research. The most common technique to analyze the composition of the gu<sup>t</sup> bacteria is marker gene sequencing, generally using the 16S rRNA gene [23–25]. Advantages of this method are related to its simplicity and low cost.

The aim of this review is to overview the most up-to-date literature about the evolving role of gu<sup>t</sup> microbiome manipulation in IBS management, with a focus on probiotics, prebiotics, antibiotics, and fecal transplantation.
